What a chip actually is
A computer chip, or integrated circuit, is a small piece of material with an enormous number of tiny electronic switches wired together. The switches are transistors. A modern processor can hold billions of them, and together they carry out the on-and-off logic that underlies everything a computer does. The details of how they are organized can be found in the history of computing; this article is about how the physical object is made.
Why silicon
Most chips are built on silicon, an element found in sand and many rocks. Silicon is a semiconductor, meaning its ability to conduct electricity sits between that of a metal and an insulator. More usefully, its conductivity can be changed on purpose. Adding tiny amounts of other elements, a step called doping, makes regions that conduct in different ways. Combining such regions lets a transistor switch current on or off depending on a voltage applied to it.
Silicon also forms a stable, insulating oxide on its surface, which helps in building layered structures. These practical advantages, along with abundance and long industry experience, explain why it dominates, though other materials are used in specialized chips. The properties that make this possible are studied in materials science.
Step one: making the wafer
The process starts with silicon refined to extreme purity. It is melted and grown into a large single crystal, which is then sliced into thin round discs called wafers and polished until nearly perfectly flat. Many chips are made on a single wafer at once, arranged in a grid, and cut apart at the end.
Step two: building layers
A chip is made by repeating a cycle of steps many times, building a three-dimensional structure one thin layer at a time. The main steps in each cycle are broadly these:
- Deposit. A thin film of material, such as an insulator or a metal, is laid over the wafer.
- Coat. The wafer is covered with a light-sensitive material called photoresist.
- Expose. Light shines through a patterned plate called a mask, projecting the circuit pattern, greatly shrunk, onto the photoresist. This is photolithography. The most advanced systems use very short wavelengths of light, which allow finer detail.
- Develop. The exposed (or unexposed, depending on the type) photoresist is washed away, leaving a pattern.
- Etch. Chemicals or plasma remove material from the areas not protected by the pattern.
- Dope or add. Impurities are introduced into chosen regions to change how they conduct, or new material is added into the etched openings.
- Clean and repeat.
A finished processor often requires many layers and hundreds of individual steps over a period of weeks. The lowest layers form the transistors; the upper layers form copper or other metal wiring that connects them, stacked in many levels like a multi-story road system.
Why “nanometer” numbers are confusing
Chips are often described by a process node with a figure in nanometers. A nanometer is one billionth of a meter. Historically the number related to a physical feature size, but today the labels are largely a marketing and generational shorthand and do not directly equal the size of any one part. A smaller number generally signals a newer, denser process, but comparing numbers across different manufacturers is not straightforward.
Smaller transistors let more of them fit in the same area, which can improve speed and efficiency, though gains have become harder and costlier to achieve.
The cleanroom
Dust that would be invisible in a normal room can ruin a circuit whose features are far smaller than a speck. Chip factories therefore operate as cleanrooms with filtered air, special garments for workers and tightly controlled temperature and humidity. Wafers are usually moved in sealed containers and handled largely by machines.
Even so, defects happen. Not every chip on a wafer works. The fraction that does is called the yield, and it is a major factor in cost. Larger chips are more likely to include a defect, which is one reason very large processors are expensive to make.
Testing, cutting and packaging
When the layers are complete, each chip on the wafer is tested electrically. The wafer is then cut into individual pieces, called dies. Working dies are mounted in protective packages with tiny connections to the outside world, sometimes with several dies combined in a single package. Further testing sorts chips by performance; chips from the same wafer may end up sold as different products depending on how well they run.
A global and specialized supply chain
Chip making is divided among specialist businesses. Some companies design chips, others operate the factories, and others make the machines, chemicals and software the factories depend on. A single chip may cross several borders during design, fabrication, packaging and testing. Building a modern factory takes years and very large investment, which is why shortages are slow to fix and why the industry draws attention from governments. Details of any particular company or country’s share change over time and are best checked in current sources.
Key points
A chip begins as ultra-pure silicon sliced into wafers. Repeated cycles of deposit, pattern with light, etch and dope build transistors and wiring layer by layer, in spotless factories, with the finished dies tested, cut and packaged. The difficulty comes from doing this at near-atomic scale, repeatedly, with very few defects.